What is wrong with my dog?

my dog is a male, 4 years old. we never have him around other dogs because he just tries to eat them. the other day he was outside with me and attacked this other gross little dog. thankfully my boyfriend got him before he ate the other dog. the other dog is gross, he has patches, hes dirty and is always running around by the house now. the other day someone left my dog outside by himself for an hour. i guess he was playing with the other little dog and now he wont even hang out or be normal. hes hyper all day just looking outside the windows and by the door. and when the other dog comes by, he crys. whats wrong with him?! hes not my dog anymore!